Base material for producing blades for circular saws, cutting-off wheels, mill saws as well as cutting and scraping devices

ABSTRACT

The base material consists of a base steel having a base carbon content of less than 0.3 wt % carbon and enriched with carbon starting from its surface  2,3,4  formed of formed of two broad surfaces  2 , two short edge sides  3  and two long edge sides  4 . The base steel has boundary areas  5  enriched with 0.5-1.1 wt % carbon starting from at least one broad surface  2  and transforming with a decreasing carbon content into an area  6  not enriched with carbon or only slightly enriched. At the edge surface  3,4 , the base material has the sandwich structure formed of the carbon-enriched boundary area  5  and the area  6  not enriched with carbon. (FIG. 1)

The invention pertains to a base material for the production of blankblades, particularly for circular saws, cutoff wheels, gang saws, aswell as for cutting and scraping devices, consisting of a base steelenriched with carbon starting from its surface consisting of two broadsurfaces, two end face surfaces and two long edge surfaces, wherein thebase steel has a basic carbon content of less than 0.3 wt % carbon.

It is conventional to use tool steels with a carbon content between0.5-1.0 wt % or low-alloyed structural steel (as steel for tempering) inorder to produce a base material for the production of blank blades,particularly for circular saws, cutoff wheels, gang saws, as well as forcutting and scraping devices. The heat treatment of these materials isthen done with the objective of obtaining a homogeneous texture and auniformly high hardness over the entire thickness range. The necessarytoughness of the base materials is achieved by a controlled tempering,the latter, however, necessarily being connected to losses of hardness.Depending on the purpose of use and the specific load on the basematerial, for saws, for instance, hardness values between-roughly 37-50HRC are produced.

Particularly in the hot-rolling process of a typically used tool ortempering steel and in the austenitization treatment of it forhardening, the carbon diffuses out of the boundary layer of thematerial. A decarbonization of the surface results, so that thedecarbonized boundary layer with low hardness has to be ground awayafter heat treatment.

In order to improve service life, a large number of saws arehard-chrome-plated, tipped with hard metal or diamonds or stellitized.The tipping is done by soldering or sintering. These measures lead toclear improvements of service life without, however, influencing theinherent strength of the blank blades. The manufacturing costs of thesesaws are markedly increased by the measures for increasing service life.This necessarily leads to a reduction of the teeth or number ofsegments, which worsens the cutting quality and increases the noiseemission.

In the corporate publication “Sie+Wir” of the Stahlwerke Südwestfalen,No. 14/1975, manufacturing processes for various types of saws aredescribed, reference being made to the fact that there is always ademand for a sheet which is as free of strains as possible with lowdecarbonization values and homogeneous texture formation. The steelsused must have a very fine-grained texture with good tenacity afterhardening and tempering, so that the very high centrifugal and shearingforces that appear can be securely absorbed.

The typification of the saws in the aforementioned corporate publicationrelies on a customary division into three groups, corresponding to thematerial to be cut. According to the material group, differentrequirements are placed on the properties of the saws. These groups are:

1. saws for wood and plastic (circular wood saws, hard metal tippedcircular saws, forestry and gang saws;

2. saws for metal (segmented circular saws, cutoff saws, circular hotsawing machines);

3. saws for stone (diamond-tipped circular saws, diamond-tipped slabsaws).

One of the requirements of saw blades is the presence of a high bendingstiffness or shape stability. To stabilize slab, band, circular, andquick-cutting saw blades as well as diamond discs, in particular tocompensate for strains produced by nonuniform heating in the tool body,a known procedure consists in producing internal strains in certainzones deliberately by tensioning the blade (“Comparative studies on thetensioning of circular saw blades with machines and flattening hammers,”in the special issue of Holz als Roh- und Werkstoff, Vol. 21 (1963), pp.135-144). Such a generation of internal strains can be accomplished inhardened steel disks or bands by cold hammering with a hammer ormechanically by rolling or pressing, but in any case, it represents anelaborate processing step in manufacturing.

The thermochemical enrichment of iron and steel materials with carbonhas been known for some time, and is referred to as case-hardening; Ifnitrogen is introduced into the material at the same time, one speaks ofcarbonitriding. An overview of caburizing, with special emphasis inregard to a mathematical modeling of it, is provided, for instance, bythe article “The carburizing process” in Härterei TechnischeMitteilungen, Vol. 50 (1995) No. 2, pp. 86-92. The carburizing processcan take place in a gaseous medium, in a salt bath or in powder and isgenerally performed at temperatures between 900-1000° C. As carbondonors, agents are employed here whose carbon activity must be higherthan that of the iron material. The carbon emitted from the carburizingagent diffuses into the boundary layer of the workpiece to becarburized. A characteristic carbon concentration profile results,according to the selected process parameters, such as temperature andtreatment time, as well as the carbon activity of the carburizing agentand the composition of the iron material. The carbon concentrationdeclines continuously with increasing distance from the boundary, untilit reaches the initial level of the material in the inside of thematerial. The carburizing depth A_(t) is to be considered acharacteristic parameter of significance for practice in this regard.The carburizing depth A_(t) is defined as the vertical distance from thesurface up to a boundary characterizing the thickness of the layerenriched with carbon. The carbon content at which this boundary isassumed to exist is subject to standardization (cf. DIN EN 10 052) andis generally agreed to be 0.35 wt % carbon. The carburizing depth A_(t)of a material increases with increasing duration of carburizing of aworkpiece, the geometry of the latter also playing a role. Forconvex-curved workpiece surfaces, at edges or points, therefore, agreater carburizing depth A_(t) occurs, since a comparatively smallervolume-is available to the carbon diffusing in from all sides. Therebyan excess carbonization can occur, which is characterized by theseparation of carbides or by an undesired residual austenite contentafter hardening.

A method of this class for producing highly alloyed strip steel which isused for quick-cutting and tool steel as used for, among other things,the purpose of manufacturing blades and cutters found in razor blades ormetal saw blades, has become known from DE-OS 2,431,797. The highcontent of alloy elements and the type of alloy elements, e.g. 12-13 wt% chromium, whereby a high hot hardness can be achieved, corresponds tothis purpose of the strip steel for metal saws or razor blades,classified in the second group-according to the division above. Highlyalloyed steels with additional high carbon content are difficult toprocess using hot and cold rolling in the manufacturing process, i.e.,they are at risk for cracking and fracturing. Therefore a strip materialwith low carbon content is first either sintered or cold-rolled andsubsequently enriched with carbon, either over its entire surface orpartially, in the edge area. The carbon enrichment is done over theentire cross section or thickness of the strip material. Thus a carbonconcentration corresponding in its level to the carbon concentration oftool steels results with almost a constant profile over the entirethickness of the strip material, slight corresponding to the foreseenusage of the material.

From AT-PS 372,709, a cutting tool, specifically a saw, made of alloyedsteel is known, which is enriched in the area of its working surfaces orteeth with 1.8-2.2 wt % carbon to a depth of 0.02-0.10 mm, the carboncontent at a depth of 0.15-0.25 mm reaching the carbon content. Thesteel alloy consists of iron with the unavoidable impurities andcontains 0.1-0.3 wt % carbon, 0.2-2.0 wt % silicon, 0.5-1.5 wt %manganese, 5.0-7.0 wt % chromium, 1.0-2.0 wt % tungsten, 1.0-2.0 wt %molybdenum, 0-2.0 wt % vanadium, 0-0.5 wt % titanium, and 0-0.5 wt %niobium. To produce the cutting tool, the workpiece blank, specifically,the saw blade, is subjected to a case-hardening at temperatures in therange of 850-1050° C., whereafter the hardening in air, oil or in a hotbath takes place. The slight carburization depth A_(t) and the strongcase hardening lead to a carbon gradient from the broad surface to thearea not enriched with carbon of roughly 6-14 wt % C/mm in the boundaryarea of the base steel. In this way it is intended, in particular, for asurface layer of elevated wear resistance to be achieved. The alloyemployed is a special steel, corresponding by its content-of alloyelements to a high-speed steel, but without having a correspondinglyhigh carbon content. The carbon content here is typical of carburizedsteels , but the alloy content is atypical. The use of such a materialpursues the goal of replacing fast-machining-steel by the alloyspecified and treated in the manner described. Here too, similar to themethod corresponding to DE-OS 2,431,797, a reduction of themanufacturing costs by reducing the risk of rejects and a savings inmaterial by avoiding an overuse of strip steel in its forming processesis intended. In the process, a high hot hardness in the workpiece can beachieved, which is characterized by tempering temperatures of 500° C.and more. Given the core hardness of the material, a value of 45-55 HRCcan be assumed, as with fast-machining steels.

A disadvantage of this cutting tool and its manufacturing methodconsists in the fact that band saws are expressly out of the question,presumably, because the necessary tensile and reversed bending fatiguestrength cannot be achieved. As workpiece blanks, moreover, keyhole sawblades are produced by stamping, milling and setting the teeth, and areonly thereafter case-hardened, hardened and tempered. It must be assumedhowever, that after this treatment the saw blades can no longer havetheir teeth set, because of their high carbon content. Because of thecase hardening taking place omnidirectionally, moreover, an excessivecarbon enrichment may occur in certain edge areas, as described above,which has an unfavorable effect on the edge properties and strength ofthe teeth because it causes embrittlement of the material.

The invention is based on the problem of specifying a base material ofthe generic type with which blank blades for circular saws, cutoffdisks, gang saws and cutting and scraping devices with enhancedcomponent strength can be produced while avoiding the formation of adecarbonized edge area, wherein a higher hardness at the surface ispossible to increase wear resistance with identical operating andfracture safety and the noise emission in the operating state isreduced. It should furthermore be possible for untipped saws for woodand plastic, such as circular wood saws, forestry or gang saws to beproduced from this base material, which are distinguished by a longservice life with low production expense.

This problem is solved by a base material of the generic type in whichthe base steel has boundary areas enriched by a thermochemical treatmentwith 0.5-1.1 wt % carbon starting from a least one outside surface andmaking a transition with decreasing carbon content to an area notenriched at all with carbon, or only enriched slightly, while, at theedge surfaces, the base material has the sandwich structure formed bythe boundary area enriched with carbon and the area not enriched withcarbon. The thermochemical treatment is preferably a carburizingprocess, but can also advantageously be a carbonitriding process, if thecarburizing medium contains nitrogen or nitrogen compounds, such asammonia. The nitrides formed in this manner in the base materialaccording to the invention cause an additional elevation of wearresistance and counter fatiguing of the material.

In this manner, the tool steels with a high degree of purity that areordinarily used can be can be replaced by the base material according tothe invention, whose base steel, preferably low-alloyed or unalloyedstructural steel, need not meet these purity requirements. Specialsteels are not required as starting materials, which implies a reductionof the steel manufacturing costs. With the base material according tothe invention, it is not only for an elevated wear resistance to beachieved at the broad surfaces, but also a greater component strength,characterized by a greater bending strength, static bending strength andreversed bending strength.

The base material can advantageously also have a sandwich structurewhich consists of a broad surface enriched with carbon, an inner corenot enriched or only slightly enriched with carbon and an additionalbroad surface of the base steel enriched with carbon. After theproduction of saws, cutoff disks or cutting devices, this structure isthen also present on the saw teeth or blades. With repeated use of thetool, an uneven wear results over the thickness of the material,specifically a so-called cratering. That is to say, the hard andwear-resistant broad surfaces wear more slowly than the core which isnot enriched with carbon, whereby the edge surface obtains a concaveshape and a self-sharpening effect occurs in the cutting area.

It has been shown that, since the physical properties of the basematerial can be gradually changed by differing carbon contents, it is ofparticular advantage for the wear and strength properties to be achievedin the blank blades if the quotient of a carburizing depth A_(t) of theedge area, in which the carbon content is 0.35 wt %, and the thicknessof the base material had a value of 0.15-0.40. The depth of thecarburized area can preferably be chosen such that, after hardening andtempering of the thermochemically treated base material, at most roughly⅓ of the total depth of the base steel has essentially the originalhardness of the base steel or a slightly higher hardness, and at leastroughly ⅔ of the thickness of the base material has a higher hardness.In particular, it is preferred that, after hardening and tempering ofthe thermochemically treated base steel, at least roughly 50% of thethickness of the base material has essentially the original hardness ofthe base steel or a slightly higher hardness, and at least roughly 50%of the thickness of the base material has a higher hardness. Afterhardening and tempering, the hardness of the broad surfaces of the basematerial advantageously lies in the range of roughly 50-63 HRCpreferably in the range of 55-60 HRC and, in the area not enriched withcarbon, 20-40, preferably, 30-35 HRC. The enrichment of the base steelwith carbon on both sides over the entire broad surface of the steelsheet, but the carbon enrichment can also be conducted only partially,in the later toothed area of the on both sides, or partial areas, atsubsequent soldering areas or the like, can be provided which areexcluded from carbon enrichment. The areas not enriched with carbon oronly slightly enriched consist after hardening and tempering of a-mixedstructure of ferrite and perlite and/or of bainite, preferably in itslower stage.

Thus it is possible, with low requirements on the base steel for saws tobe produced which consists of a steel sheet that is enriched with carbonon both sides, for instance, or only partially, by means of athermochemical treatment, particularly carburizing. Surprisingly, it wasestablished that, with a base steel having a very low carbon content of0.1-0.2 wt % and subsequent case hardening as well as hardening andtempering, saws can be produced with better quality which have nolinear/hardness strength profile with respect to their thickness andsurface. The boundary area enriched with carbon here favorably has amean carbon gradient of roughly 0.25-0.75 wt % C/mm, preferably0.40-0.50 wt % C/mm for the surface to the area not enriched withcarbon.

While conventional saws have a martensitic structure throughout withhomogeneous properties, this is present in the saws produced from thebase material according to the invention only at the surfaces of theareas enriched with carbon. The requirements for toughness are largelymet by the softer core, while, in case of a saw that is not tipped orstellitized, the surface determines the good cutting properties and highstability of the saw with its hardness.

As already presented, low- or nonalloyed structural steels are preferredas base steels for the base material according to the invention. Thusall steels that can be used, alloyed or unalloyed, as hardened steelsare suited for the base material according to the invention.Heat-treatable steels with low carbon contents, as well as rust- andacid-resistant steels with an elevated chromium content (12-13 wt %) canlikewise be used. In Table I, such steels that can be utilized accordingto the invention are presented by way of example, without the inventionbeing limited to these, however.

TABLE I Possible base steels for the base material according to theinvention Desig- Designa- nation per DIN 17006 nation per DIN 17006Alloy type, wt % C 10 1.1121 0.10 C C 15 1.1141 0.15 C 15 Cr 3 1.70150.15 C; 0.6 Cr 16 MnCr 5 1.7131 0.16 C; 1.2 Mn; 0.9 Cr 15 CrNi 6 1.59190.15 C; 1.5 Cr; 1.6 Ni 18 CrNi 8 1.5920 0.18 C; 2.0 Cr; 2.0 Ni 25 CrMo 41.7218 0.26 C; 1.1 Cr; 0.3 Mo X 10 Cr 13 1.4006 0.11 C; 13 Cr

The invention is explained in further detail on the basis of severalexamples with reference to the attached drawings. These show in:

FIG. 1, a view in perspective of a plate of a base material according tothe invention for the production of blank blades for circular saws,cutoff disks, mill saws and cutting and scraping devices;

FIG. 2, the comparative presentation of the carbon concentrationprofiles of three qualities of the base material according to theinvention, produced using various types of steel as the base steel;

FIG. 3, the comparative presentation of the hardness profiles of thebase materials according to the invention from FIG. 2;

FIG. 4, the comparative presentation of the static bending stiffness ofa conventional base material made of hardened tool steel and of a basematerial according to the invention for differing sheet thicknesses; and

FIG. 5, the result of a bending test on flat samples of a base materialaccording to the invention in the form of a force/deformation diagram.

In FIG. 1, a plate 1 of a base material according to the invention ofthe type that is characteristic of all embodiments described below isshown. The surface of the plate 1 is formed of two broad surfaces 2, aswell as two end face surfaces 3 and two long edge surfaces 4. As a basematerial for the blade blanks to be manufactured, plates 1 of this typeare-trimmed on the end face surfaces 3 and the long edge surfaces 4after thermochemical treatment and are supplied to the manufacturer inthis form, or the tool manufacturer stamps out or laser-cuts the desiredparts such that machining of carburized areas of the edge surfaces 3,4is avoided. According to the invention, the base material is enrichedwith carbon only going out from the broad surfaces 2 and not from theedge surfaces 3,4. As a result of the thermochemical treatment, the basematerial has boundary areas 5 enriched with 0.5-1.1 wt % carbon startingfrom the broad surface 2 and making a transition with decreasing carboncontent to an area 6 not enriched with carbon, in this case, a core area6, due to the carburization on both sides. At the edge surfaces 3,4, thebase material has the sandwich structure formed of the boundary area 5enriched with carbon and the area 6 not enriched with carbon.

In the illustration, 7 of saw blade blanks 8 a for circular saws and sawblade blanks for gang saws 8 b are indicated. To produce the plate 1 ofthe base material according to the invention, the starting point was oneof the base steels indicated below with a carbon content of less than0.3 wt % carbon.

EXAMPLE 1

Material Utilized: C 15 Cold-rolled Strip, Annealed Sample Thickness:D=2.5-2.7 mm

A thin-layer carburization was performed on several samples at atemperature between 880-930° C. and a treatment duration in the range of60-90 min in an endogenous atmosphere enriched with propane so that, ascan be seen from FIG. 2, a boundary layer 5 resulted, with a meanpenetration depth A_(t) of roughly 0.8 mm with a scattering from roughly0.6-1.0 mm. The quotient of the penetration depth A_(t) of the boundaryarea 5 of the thermochemically treated base steel and the thickness D ofthe base material took on values from 0.15-0.40 and averaged 0.32. AsFIG. 2 further shows, the carbon content directly on the broad surfaceslay between 0.7 and 0.8 wt %. The carbon-enriched boundary area 5 of thebase steel had a mean carbon gradient of roughly 0.30-0.55 wt % C/mmbroad surface 2 to the area 6 not enriched with carbon.

The subsequent hardening at a temperature in the range of 820-860° C.with oil quenching resulted with good flatness of the base materialplate 1 in hardnesses of roughly 63-65 HRC at the broad surfaces 2 and44 HRC in the core 6. After a tempering period of 3 h at a temperatureof 260°[C], found to be optimal, hardness values of 56 HRC (700 HV) onthe flat surface 2 and 40 HRC (400 HV) in the core 6 were achieved asFIG. 3 shows. The carbon enriched boundary area 5 of the base materialhad a mean hardness gradient between 9-15 HRC from the broad surface 2to the area 6 not enriched with carbon or only slightly enriched. In thepresence of such a hardness profile curve, setting of the saw teeth canstill be performed using this base material for saws. A saw producedfrom this base material is distinguished by a high stiffness and dynamicstressability, is very quiet and has a hardness roughly 10 HRC greaterthan a saw known from the prior art, but also has very good wearresistance. This base material appears to be particularly suited fornonrotating saws and for cutting and scraping devices.

EXAMPLE 2

Material Utilized: 13 CrMo 4 4 Cold-rolled Strip, Annealed SampleThickness: D=2.4-2.7 mm

A thin-layer gas carburization was conducted on several samples withprocess parameters as in the first embodiment, so that, as FIG. 2illustrates, a boundary layer 5 with a mean penetration depth A_(t) of0.7 mm resulted. The quotient of the carburizing depth A_(t) of theboundary area 5 of the thermochemically treated base steel and thethickness D of the base material took on mean values of about 0.25. AsFIG. 2 also shows, the carbon content directly on the broad surfaces 2was roughly 0.7 wt %. The carbon-enriched boundary area 5 of the basesteel had a mean carbon gradient of roughly 0.46-0.53 wt % C/mm from thebroad surface 2 to the area 6 not enriched with carbon.

The subsequent hardening, likewise conducted essentially under the sameconditions as in the first embodiment, Led to hardness values on theboard surfaces 2 and the core 6, respectively, differing only slightlyfrom the first embodiment, with good flatness of the plate 1. After atempering period of 3 h at a temperature of 300° C., determined to beoptimal, hardness values on the broad surface 2 of roughly 54-55 HRC(ca. 670 HV) and in core 6 of roughly 38 HRC (380 HV) were achieved, asFIG. 3 shows. The carbon-enriched boundary area 5 of the base materialhad a mean hardness gradient of roughly 15 HRC/mm from the broad surface2 to the area 6 not enriched with carbon or only slightly enriched.

The base material of this embodiment of the invention appearsparticularly suited to the manufacture of table and trim saws ascircular saws with set teeth of roughly 55 HRC. The hardness forordinary saws of this type of manufactured tool steel is about 43-44HRC.

A circular saw blade was produced from the base material according tothe invention in order to determine the static bending strength C. Thestatic bending strength C of the saw blade results as the quotient of abending force F exerted in the static load case under defined conditionsand a deformation f appearing at the load point. The saw blade had thediameter dimension D_(K) and the thickness D listed for I in Table II.The diameter D_(I) of an interior circular opening of the saw blade was40 mm. The saw blade was clamped with a flange having a diameter D_(E)of 118 mm. Thus there was a characteristic ratio of clamp and sawdiameters D_(E)/D_(K) with a value of 0.34. The measurement points atwhich the bending force F was exerted and at which the deformation f wasmeasured were on a measuring circle 95 mm away from the outer edge ofthe flange. The bending force was 19.7 N and was measured at four pointsof the measurement circle on the front and back side of the saw blade.

TABLE II Saw blade dimensions Saw Blade No. Diameter D_(k) in mmThickness D in mm I 330 2.4 II 350 2.5 III 351 2.5

A mean static bending stiffness C of 143 N/mm was determined, which iscontained in Table III in comparison to the values of the thirdembodiment

EXAMPLE 3

Material Utilized: 10 Ni 14 Cold-rolled Strip, Annealed SampleThickness: D=2.5-3.0 mm

A thin-layer carburization was conducted on several samples with processparameters under conditions as in the first embodiment, whereby, as FIG.2 illustrates, a boundary layer 5 with a mean penetration depth A_(t) ofroughly 0.5-0.6 mm resulted. The quotient of the carburization depthA_(t) of the boundary area 5 of the thermochemically treated base steeland the thickness D of the base material had a mean value of about 0.20.FIG. 2 also shows that the carbon content directly on the broad surfaces2 was between roughly 0.60-0.65 wt %. The carbon-enriched boundary area5 of the base steel had a mean carbon gradient of roughly 0.48 wt % C/mmfrom the broad surface 2 to the area 6 not enriched with carbon. Thesegradients, low by comparison to the prior art, have the effect that notonly is high wear resistance achieved on the broad surfaces 2, but alsointegrally high strength values of the base material according to theinvention are achieved.

The subsequent hardening, conducted essentially under the sameconditions as in the first embodiment, led to slightly lower hardnessvalues than in the first embodiment, with good flatness of the plate 1.After a tempering period of 3 h at a temperature of 200°[C] as FIG. 3shows, hardness values on the broad surface 2 of up to 54 HRC (ca. 650HV) and in core 6 of roughly 31 HRC (310 HV) were achieved. Thecarbon-enriched boundary area 5 of the base material had a mean hardnessgradient of roughly 17-20 HRC/mm from the broad surface 2 to the area 6not enriched with carbon or only slightly enriched.

Tensile strength tests were conducted on six samples with dimensions of12.5×3 mm after hardening and tempering the case-hardened base steel. Amean tensile strength value R, of roughly 1550 N/mm² was determined. Bycomparison, the tensile strength of a hardened and tempered tool steelused for known base materials had a mean value of R_(m) of roughly 1600N/mm².

Impact strength tests were conducted on six additional samples withdimensions of 55 mm×10 mm×3 mm after hardening and tempering thecarburized base material. A mean impact strength value of roughly 60J/cm² was determined. The comparative tests on six samples of hardenedand tempered tool steel used for known base materials had a mean impactstrength value of roughly 52 J/cm².

These tests show that, with the base material according to theinvention, mean tensile strength values R_(m) can be achieved thatcorrespond roughly to the tensile strength R_(m) of known basematerials, that however, for the characteristic parameter of impactstrength, so important when the blades are placed under stress in thecutting process, values can be achieved which are on average 15% higherthan for hardened materials based on tool steel.

It was possible by metallographic analyses for an optimal texturalcomposition of the base material according to the invention to bedetermined at varying distances from the broad surfaces 2. Such texturalstructures are indicated schematically by four microscopic views9,10,11,12 in FIG. 3. The carbon-enriched boundary area 5 consists off atempered mixed structure (views 9,10,11). This mixed structure containsmartensite, in some cases with carbide segregations, a small amount ofresidual austenite and intermediate textures, the martensite contentinitially rising with increasing distance from the broad surfaces 2 inthe direction of the area 6 not enriched with carbon to a maximal value(view 10) and thereafter nearly disappearing in the area 6 not enrichedwith carbon. The residual austenite content or the content of mixedstructure initially decreases with increasing distance from the broadsurfaces 2 in the direction of the area 6 not enriched with carbon to alocal minimum value (view 10), slightly increases thereafter (view 11),and strongly declines sharply in an area 6 not enriched with carbon.View 12 shows a ferritic-perlitic textural structure in the core area 6of the type characteristic of the basic structure of the base steelused.

In regard to the internal strains appearing in the base materialaccording to the invention, it was possible to determine that optimalconditions are present in this sense when maximal internal pressurestrains in the range of up to 0.90 GPa, preferably between 0.40-0.75 GPaexist in a distance from the broad surfaces 2 which is less than thecarburization depth A_(t). By contrast, internal tensile strains occurin the outer boundary area 5 with a known base material produced on thebasis of tool steel. In operation of he saw, these internal tensilestrains facilitate the introduction and propagation of cracks or provokethese phenomena. Combined with the repetitive temperature changesoccurring in multiple use of the tool, this can also cause anaccelerated material fatigue.

It is additionally advantageous if, after the hardening and tempering ofthe thermochemically treated base steel, the base material has maximuminternal tensile strains up to roughly 0.60 GPa, preferably only in therange up to 0.20 GPa at a distance from the broad surface 2 which isequal to or slightly larger than the carburization depth A_(t). Withhigher internal tensile strains, hardening cracks can easily form inthis area. Thus it is of particular advantage if the internal tensilestresses again decrease with increasing distance from the broad surfaces2 and then internal compressive strains with maxima in the range up to0.30 GPa occur at a distance from the broad surfaces 2 which is greaterthan the carburization depth A_(t). Under some conditions, the internalstrain distribution in the base material can make a tensioning of sawblades with flattening hammers or machines superfluous.

The base material of this embodiment of the invention appearsparticularly suited to the production of circular saws with set teeth ofroughly 57 HRC.

In order to determine the static bending stiffness C, two circular sawblades were produced from the base material according to the invention.The static bending stiffness C of the saw blades was determinedaccording to the method described for the second embodiment. The sawblades had the diameter dimensions D_(K) and thicknesses D listed inTable II for numbers II and III. The diameter D_(I) of the internalcircular opening of the saw blades was 40 mm, as in the second example.The saw blade was clamped with a flange D_(E) having the same dimensionsas in the second example. The positions of the measuring points and thelevel of the bending force were also identical to those for the secondembodiment of the invention. The mean values obtained for bendingstiffness are contained in Table III. Differing from he values above,the tempering temperatures lay between roughly 180° C. (II) and 220° C.(III)

TABLE III Measured values of bending stiffness C Saw blade No. Bendingstiffness C in N/mm (mean values) I 143 II 147 III 142

Depending on its quality, tempering temperatures of 150-350° C. aregenerally practical for a thermochemically treatedand hardened basesteel, taking into account its tempering resistance. The texturalstructure and the physical properties of the base material, such as thehardness profile curve, can be influenced by the tempering temperatureand time, apart from the technical parameters of the thermochemicaltreatment and hardening. Thus, hardness values of roughly 57-58 HRC weremeasured on the surface of these samples.

On the basis of the values contained in Table III and of additionalvalues obtained, the profiles of the static bending stiffness C of aconventional base material made of hardened tool steel and of a basematerial according to the invention for a characteristic ratio ofclamped to saw diameter D_(E)/D_(K)=0.34 are presented comparatively fordifferent sheet thicknesses in FIG. 4. It is seen that 1½-2 times thebending stiffness C of conventional saw blades can be achieved for sawblades with the base material according to the invention.

FIG. 5 shows the results of a three-point bending test on flat samples15 mm wide and with a thickness D of 2.8 mm made from a base materialaccording to the invention prepared according to the third embodiment.The bearing spacing of the samples was 30 mm. The illustrationreproduces a force-deformation diagram obtained from 1000 measuredvalues. As the curve profile illustrates, the maximum bending force F isachieved at a deformation f of roughly 2.00 mm after exceeding theelasticity limit at a deformation f of roughly 0.75 mm at roughly 810daN. The maximum bending strain that occurs is roughly 305 daN/mm². Fordecreasing bending force F thereafter, an additional deformation of thesamples can be observed, which indicates that the fracture occurring ata deformation f of roughly 3.75 mm is not a shearing, but rather adeformation fracture. Such a fracturing behavior of the base materialaccording to the invention offers a “set-aside opportunity” for sawblades and so forth produced from it, that is, an exchange can be madebefore the fracture appears, which increases the operating safety.

In summary, saws, cutoff disk and so forth made from the base materialaccording to the invention offer the following advantages versus thoseknown from prior art:

Due to the uniformly introduced carbon, the tools can be produced withhigh reproducibility of their properties.

The previously inevitable decarbonization in hot rolling and hardeningcan be compensated for, which eliminates regrinding of the broadsurfaces. In cold rolling, the desired material thickness can bespecified while bearing in mind the changes in dimensions occurring inthe thermochemical treatment.

Higher hardnesses of the tool on the surface with identical operatingsafety can be achieved by a targeted thermochemical treatment andpossibly by subsequent heat treatment, due to the graduated structure.

A hardness structure with fine-grained structure can be achieved afterthe thermochemical treatment of the base steel by quenching. In thatway, the subsequent hardening process can be eliminated or the physicalproperties can be improved even further by a double hardening.

By appropriate selection of the treatment parameters in thethermochemical treatment, hardening and tempering there are a number ofdegrees of freedom for generating the carbon profiles, harness profilecurves and distributions of internal strain and structure, an inconsequence the desired component properties.

The heat crack formation of saws is reduced both in the cutoff processof red-hot profiled steel as well as for-temperature increases at highcircumferential velocities in the machining of metal., especially inso-called fusion cutting.

Due to the low carbon content in the core, the danger of a hardeningwhich could be hazardous to the safety of the operating personnel isreduced in case of unwanted introduction of heat.

Compressive strains at the surface can be created in structuraltransformation by the differing structures of surface and core and theassociated change of volume during hardening and tempering. Accordingly,a strong but controlled inhomogeneity results, particularly in regard tothe internal strain state of the saws, and has positive effects on usageproperties, particularly in regard to a delayed material fatiguing and alesser susceptibility of the surface to cracking.

The component strength can be increased integrally by the base materialaccording to the invention. Thus the bending vibrations occurring inusage are reduced, particularly at high speed. A reduction of noiseemission is the consequence. Al previous measures for reducing the noiseemission of saws remain unaffected by the invention and can beadditionally utilized.

The attenuation properties if mixed structures are better than those ofpure martensite. An additional noise reduction results.

Sheet thickness can be reduced because of the higher component strength.A reduction of the cutting losses and thus a savings of material to becut results from this, due to a smaller possible cutting gap.

For a constant sheet thickness, it is possible, due to the thencomparatively stiffer blade to operate at higher cutting rates in therange of 25-75 m/min, whereby the cutting performance is significantlyincreased.

A certain substitution of the previously utilized tipped and stellitizedsaws or the hard chromium-plated gang and circular saws is thenpossible, due to the higher hardness of saws that can be achieved.

Due to the nonuniform hardness profile perpendicular to the cuttingdirection (sandwich structure), a toothed saw wears at differing ratesacross its surface. Thereby, a certain “self-sharpening effect” canresult. Advantages in resharpening saws can also be noted.

A “set-aside opportunity” is provided for saw blades produced from basematerial according to the invention because of the appearance ofdeformation fracture mechanism, which increases the operating safety.

High carbon contents, which are a disruption in the vicinity ofsoldering and welding sites, can be avoided by partial thermochemicaltreatment. This is a considerable advantage in the field of metalmachining.

Because of the softer core of the saws, it is possible to produce aso-called upset tooth by inserting a wedge. This was previously possibleonly for nickel steels.

Reference symbols  1 Plate of base material  2 Broad surface of 1  3Short edge surface of 1  4 Long edge surface of 1  5 Boundary area  6Area not enriched with carbon  7 Contour of 8a, 8b  8a Saw blade blank,circular saw  8a Saw blade blank, gang saw  9 Microscopic view in 5 10Microscopic view in 5 11 Microscopic view in 5 12 Microscopic view in 6A_(t) Case-hardening depth C Static bending stiffness D Thickness of 1D_(E) Clamped diameter D_(I) Inside diameter D_(K) Saw diameter FBending force f Deformation R_(m) Tensile strength

What is claimed is:
 1. Base material for producing blank blades forcircular saws, cutoff disks, gang saws as well as cutting and scrapingdevices, comprising a plate with two broad surfaces, two short edgesides and two long edge sides and consisting of a thermochemicallycarbon-enriched base steel plate having a base carbon content of lessthan 0.3 wt % carbon, said base steel plate having at least one boundaryarea enriched with 0.5-1.1 wt % carbon starting from at least one ofsaid broad surfaces, characterized in that: said boundary area beingenriched with carbon transitions with a decreasing mean carbon gradientof 0.25-0.75 wt % C/mm into an area not enriched with carbon; the basesteel plate being hardened and tempered so that said boundary areaenriched with carbon has a hardness of 50-63 HRC and transforms with adecreasing mean hardness gradient of 10-22 HRC/mm into said area notenriched with carbon, having a hardness of 20-40 HRC; and said edgesides of said base steel plate have a sandwich structure formed of saidcarbon-enriched boundary area and said area not enriched with carbon. 2.The base material according to claim 1, characterized in that said basesteel plate has carbon-enriched boundary areas starting from both ofsaid broad surfaces, whereby at said edge sides said base steel platehas a sandwich structure formed of said two carbon-enriched boundaryareas and said area not enriched with carbon.
 3. The base materialaccording to claim 1, characterized in that said base steel plate insaid boundary area is carburized for carbon-enrichment.
 4. The basematerial according to claim 1, characterized in that said base steelplate in said boundary area is carbonitrided for carbon-enrichment. 5.The base material according to claim 1, characterized in that said basematerial of the steel plate is an unalloyed construction steel.
 6. Thebase material according to claim 1, characterized in that said basematerial of the steel plate is a low-alloyed construction steel.
 7. Thebase material according to claim 1, characterized in that a quotient ofa carburization-depth of said carbon-enriched boundary area, at whichthe carbon content is 0.35 wt %, and the thickness between said twobroad surfaces of said base steel plate, has a value of 0.15-0.40. 8.The base material according to claim 1, characterized in that at most50% of the thickness of the base material between said two broadsurfaces has the original hardness of said steel before thethermochemical treatment, the hardening and the tempering and at least50% of the thickness of the base material between said two broadsurfaces having a higher hardness.
 9. The base material according toclaim 1, characterized in that at most ⅓ of the thickness of the basematerial between said two broad surfaces has the original hardness ofsaid steel before the thermochemical treatment, the hardening and thetempering and at least ⅔ of the thickness of the base material betweensaid two broad surfaces having a higher hardness.
 10. The base materialaccording to claim 1, characterized in that said carbon-enrichedboundary area of said base steel plate has a mean carbon gradient of0.40-0.50 wt % mm from the broad surface to the area not enriched withcarbon.
 11. The base material according to claim 1, characterized inthat said carbon-enriched boundary area of said base steel plate has amean hardness gradient of 14-18 BRC/mm from said broad surface to saidarea not enriched with carbon. 12.The base material according to claim1, characterized in that said carbon boundary area of said broad surfacehas a hardness of 52-55 HRC and said area not enriched with carbon has ahardness of 30-35 HRC.
 13. The base material according to claim 1,characterized in that at a distance from said broad surfaces, which isless then a carburization-depth of said carbon-enriched boundary area,at which the carbon content is 0.35 wt %, said carbon-enriched boundaryarea has maximum compressive residual stresses in the range between0.40-0.75 GPa.
 14. The base material according to claim 1, characterizedin that at a distance from said broad surfaces, which is equal to acarburization-depth of said carbon-enriched boundary area, at which thecarbon content is 0.35 wt %, said carbon-enriched boundary area hasmaximum tensile residual stresses in the range up to 0.60 GPa.
 15. Thebase material according to claim 1, characterized in that at a distancefrom said broad surfaces, which is greater than a carburization-depth ofsaid carbon-enriched boundary area, at which the carbon content is 0.35wt %, said carbon-enriched boundary area has maximum compressiveresidual stresses in the range up to 0.30 GPa.
 16. The base materialaccording to claim 1, characterized in that said carbon-enrichedboundary area consists of a mixed micro-structure, which containsmartensite, a slight portion of at least one constituent selected fromthe group consisting of (a) residual austenite and (b) intermediatestage microstructure.
 17. The base material according to claim 1,characterized in that said base steel plate is thermochemicallycarbon-enriched only partially in the area of said broad surfaces. 18.The base material according to claim 1, characterized in that areaswhich are not enriched with carbon consist of at least one constituentselected from the group (c) mixed ferritic-perlitic microstructure and(d) bainite.
 19. The base material according to claim 16, characterizedin that the martensite contains carbides.
 20. The base materialaccording to claim 16, characterized in that with increasing distancefrom said broad surfaces in the direction towards said area not enrichedwith carbon, the martensite content initially increases to a maximumvalue and thereafter decreases to nearly zero in said area not enrichedwith carbon, and at least one constituent selected from the groupconsisting of residual austenite content and intermediate stagemicrostructure initially decreases to a certain value, thereafterincreases and finally decreases, in said area not enriched with carbon,below said certain value.